Accurate sensors to monitor temperature are used for thermal management of micro-processors. Most System-on-Chip (SoC) solutions use some kind of temperature tracking, in order to optimize or control the performance of certain functions. At times, where phones merge with mobile computers, relevant applications of thermal sensors also include mixed-signal functions, like RF (radio frequency) or audio power amplifiers.
Most integrated solutions for accurate sensors are based on the temperature characteristic of bipolar junctions, namely the base-emitter voltage of parasitic bipolar junction transistors (BJTs). They include an analog-to-digital-converter (ADC) and a reference (e.g., bandgap), typically together with techniques for error correction. FIG. 1 is a conventional BJT based thermal sensor 100. Thermal sensor 100 includes a first PNP BJT Q1 with base and collector terminals connected to ground (Vss), and emitter terminal connected to a first current source Ic (which is coupled to power supply Vdd). Thermal sensor 100 further includes a second PNP BJT Q2 with base and collector terminals connected to ground (Vss), and emitter terminal connected to a second current source to provide current which is multiple of current Ic i.e., Ic×m, where ‘m’ is a multiple. The emitters of both Q1 and Q2 have voltages VBE1 and VBE2, respectively, which are provided to the ADC.
A conventional approach for a temperature measurement uses a voltage which increases linearly with temperature. This so-called PTAT (proportional-to-absolute-temperature) voltage is typically generated from the difference of two base-emitter voltages (i.e., ΔVBE) of PNP BJTs Q1 and Q2, which are biased with different current densities (e.g. ratio 1:m, where ‘m’ is an integer or fraction). An actual temperature value can be extracted by comparing the PTAT voltage (=“ΔVbe”) to a temperature independent reference (e.g., a bandgap). In practice, this is achieved by measuring the PTAT voltage (or a multiple of it) directly with an ADC, which in turn includes (or is controlled by) such reference. These circuits may achieve high precision after trimming, but the solution is very complex. Multi-placement is therefore costly.
Multiple hot-spot sensing using conventional thermal sensors can be performed in deep-submicron technologies and may achieve a significantly smaller size e.g., by using remote diodes away from the core thermal sensor. However, the absence of error compensation results is reduced accuracy, with still average power consumption. Some alternative (non-BJT) concepts yield more “digital-alike” circuits, e.g., by using frequency of ring oscillators as thermal reference. But the assumed advantage towards technology scaling manifests actually as a handicap, because the characteristics of MOS (metal-oxide semiconductor) devices vary strongly with process. Poor linearity and spread is discouraging for using such concepts in volume production.
Calibrating a thermal sensor is also a challenge. Calibrating thermal sensor 100 requires a one-point or two-point calibration process, with precise control of die temperature during test. It is difficult to establish a precise temperature on a die for trimming a thermal sensor during test. Current solutions require large effort for calibration control, with yet impaired accuracy, due to temperature uncertainty.